The present invention pertains generally to radar antennas. In particular the present invention pertains to ground-based radar systems that incorporate airborne antennas. More particularly the present invention pertains generally, but not exclusively, to target acquisition systems which employ airborne antennas that are supported by aerostats.
Several considerations must always be addressed during the design and development of any effective radar system. In particular, and of special concern for the present invention, is the configuration of an antenna that can be used for a ground based radar system, and the way in which it is to be operationally deployed with an aerostat. For this concern, both technical and operational considerations need to be addressed. For example, technical consideration that can affect the target acquisition capability of a radar antenna include its size, its rigidity, its ability to direct a transmitted radar beam along a desired beam path and, of course, its power. Further, important operational considerations involve the location of the antenna, its steerability and, depending on its mission profile, the ease with which it can be set up for deployment and dismantled for subsequent relocation and redeployment.
It happens that target acquisition radar systems require effectively unobstructed line-of-sight beam paths. Thus, radar systems, in general, are adversely affected by xe2x80x9cclutterxe2x80x9d in the form of unwanted echoes from terrain features and man-made structures in the immediate vicinity of the antenna. Accordingly, for ground-based radar systems, an obvious solution is to somehow elevate the radar antenna.
Towers, or other types of vertical structures, are quite commonly used for the purpose of elevating radar antennas to a location where they can be effective. For situations wherein a relatively high degree of mobility is required, however, it may be more cumbersome and time consuming to erect and dismantle antenna towers than is operationally warranted. In such situations, it has been proposed that an aerostat be used as a platform for the antenna. The use of an aerostat for this purpose, however, introduces additional considerations of antenna weight which would otherwise be of much less concern. For instance, the necessary rigidity for an antenna is typically provided by a structure which, even when made of a relatively lightweight material, still has substantial weight. Also, because transmit apertures for antennas are heavier than their associated receive apertures, it may be desirable to reduce the size, and consequently the weight, of the transmit aperture for an aerostat based antenna. The result of such an antenna configuration is that the transmit beamwidth effectively grows larger (i.e. a xe2x80x9cfloodlightxe2x80x9d beam). Consequently, because target detection probability remains a function of energy on target, there is a diminution in target detection ability.
In light of the above, it is an object of the present invention to provide an airborne radar antenna system for detecting a target in a volume that includes an antenna made of a light weight material, such as printed circuits on a flexible mylar sheet. Another object of the present invention is to provide an airborne radar antenna system for detecting a target in a volume that is capable of effectively using a smaller transmit aperture than its receive aperture. Still another object of the present invention is to provide an airborne radar antenna system for detecting a target in a volume that can be effectively deployed with an inflatable aerostat. Yet another object of the present invention is to provide an airborne radar antenna system for detecting a target in a volume that is easy to use, relatively simple to manufacture, and comparatively cost effective.
In accordance with the present invention, an airborne radar antenna system for detecting a target in a volume includes at least one inflatable aerostat, and a same number of tethers that respectively anchor each aerostat to points on the ground. A radar antenna, for transmitting and receiving a radar beam, is supported by each aerostat at respective locations above ground level. It is contemplated for the present invention that the radar antenna is preferably one square meter in size and less than approximately seventy kilograms (70 kg). Importantly, the radar antenna may be made of a flexible material. For example, the antenna can be made of a flexible sheet on which the required antenna elements have been printed. The flexible sheet can then be mounted on a rigid frame which, in turn, is supported by the aerostat. Thus, the antenna can be supported by the aerostat in any of several ways. These include mounting the antenna inside the buoyancy chamber of the aerostat. Alternatively, the antenna can be mounted on the surface of the aerostat""s buoyancy chamber or in an enclosure that is suspended beneath the aerostat.
Included in the system of the present invention is a ground-based transmitter that is positioned at a distance from the aerostat. The specific purpose of this transmitter is to radiate a beacon signal toward the antenna at the aerostat. A computer is then used to evaluate the beacon signal as it is received by the antenna for purposes of creating an error signal. Importantly, this error signal is indicative of any deviations or distortions that may be experienced by the flexible antenna from its desired configuration. Accordingly, with this error signal, system mechanisms can then be activated to electronically or mechanically reconfigure or calibrate the antenna element, as necessary, to orient and direct the radar beam along a predetermined beam path toward the volume. Additionally, system mechanisms can be incorporated for rotating or spinning the antenna element to sweep the radar beam through the volume.
Also included in the system of the present invention is a ground station that is established to house the computer and any other subsystems that are required to control the antenna for its target acquisition mission. In order to affect this control, a communications link is provided that connects the computer and other subsystems at the ground station with the antenna at the aerostat. The present invention contemplates that any communication, whether it is a two-way or one-way communication, between the ground station and the antenna, can be established through the communications link. By way of example, DC power from a power source at the ground station can be sent, through the communications link, and to the antenna for transmitting and receiving a radar beam. Preferably, the communications link is an optical fiber that is incorporated with the tether. The communications link, however, may be a wireless or an optical link of any type known in the pertinent art.
It can happen that for certain applications, it is desirable, or necessary, for the receive aperture of the antenna to be a different size than the antenna""s transmit aperture. If so, for instances wherein a first aperture (of area A1) is used for transmitting the radar beam, and a second aperture (of area A2) is used for receiving a return signal from said radar beam, and wherein A1=nA2 with n greater than 1, the present invention envisions filling the transmitter beam with multiple-simultaneous receive beams and having an appropriately increased dwell time on the return signal for target detection by the receive aperture (A2). Specifically, if xe2x80x9cxxe2x80x9d seconds are required to detect the target for an antenna configuration wherein the value of xe2x80x9cnxe2x80x9d is one (n=1), the system of the present invention contemplates increasing the dwell time of the antenna to xe2x80x9cnxxe2x80x9d seconds for receiving the return. The result is an equivalent volumetric search rate with a reduced transmit aperture resulting in reduced size, weight, and (relative) cost.